Implement top_metrics agg (#51155)

The `top_metrics` agg is kind of like `top_hits` but it only works on
doc values so it *should* be faster.

At this point it is fairly limited in that it only supports a single,
numeric sort and a single, numeric metric. And it only fetches the "very
topest" document worth of metric. We plan to support returning a
configurable number of top metrics, requesting more than one metric and
more than one sort. And, eventually, non-numeric sorts and metrics. The
trick is doing those things fairly efficiently.

Co-Authored by: Zachary Tong <zach@elastic.co>